According to The Guardian (reporting an analysis by WWF) humanity stands on the verge of an ecological catastrophe as:

Humanity has wiped out 60% of mammals, birds, fish and reptiles since 1970, leading the world’s foremost experts to warn that the annihilation of wildlife is now an emergency that threatens civilisation.

While catastrophic declines in some species are well known (from overfishing, for example) this seems like a big claim. And even overfishing has been contained and even reversed by tighter regulation.

@matt_black: "Overfishing has been contained and even reversed by tighter regulation." -- Do you have citations for that (not from the fishing industry)?
– DevSolarOct 30 '18 at 11:51

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While the headline of the Guardian article mentions "animals" the body of the article makes it clear the claim is limited to veterbrates (animals with a backbone). Hat-tip to @DevSolar for mentioning insects in particular.
– Oddthinking♦Oct 30 '18 at 12:15

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@matt_black: "...*not* from the fishing industry", and you link to seafish.org? :-D I also note that the linked article says a lot about how cod population had decreased, that the EU has taken action to protect it, and that the (much criticised) MSC has now "fully certified [North Sea cod] as sustainable" -- but nothing about how the cod population has developed. For all we know (from that source), the MSC has just buckled in to lobby pressure.
– DevSolarOct 30 '18 at 12:30

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As I see it, you don't need to discuss whether overfishing has been reversed. If you simply have the citation from the Guardian be followed by your next sentence ("While catastrophic declines… like a big claim."), you leave out a potentially distracting and controversial point without impairing the quality of your question in any way.
– SchmuddiOct 30 '18 at 13:01

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I feel like "overfishing has been contained and even reversed by tighter regulation" should be split out into its own question. Illegal, Unreported and Unregulated fishing is a huge problem, and I'd be amazed if any sources in the paywalled article properly account for it.
– user568458Oct 31 '18 at 9:54

4 Answers
4

To get a littler closer to the horse's mouth -- the news article is about the Living Planet Report 2018 as published by the WWF (the wildlife people, not the wrestling people.)

On page 7 of that report we have:

The Living Planet Index also tracks the state of global biodiversity by measuring the population abundance of thousands of vertebrate species around the world. The latest index shows an overall decline of 60% in population sizes between 1970 and 2014.WWF Living Planet Report 2018 page 7

The Living Planet Index is described in further detail in the same report:

Living Planet Indices – whether the Global Index or those for a specific realm or species
group – show the average rate of change over time across a set of species populations. These
populations are taken from the Living Planet Database, which now contains information on
more than 22,000 populations of mammals, birds, fish, reptiles and amphibians. The global
LPI is based on just over 16,700 of these populations. This is because some populations
overlap in both space and time, so to avoid double-counting, certain populations are not
included when calculating a global trend.WWF Living Planet Report 2018 page 90

And the index values themselves

represent the average change in population
abundance – based on the relative change and not the
absolute change – in population sizes. The shaded areas
show 95% confidence limits. These illustrate how certain
we are about the trend in any given year relative to 1970.
The confidence limits always widen throughout the timeseries
as the uncertainty from each of the previous years
is added to the current yearWWF Living Planet Report 2018 page 90

Note that the report itself doesn't causally ascribe this to humans. But it does claim that overexploitation (a human action) is a main driver:

While climate change is a growing threat, the main drivers of
biodiversity decline continue to be the overexploitation of species,
agriculture and land conversion. Indeed, a recent assessment found
that only a quarter of land on Earth is substantively free of the
impacts of human activities. This is projected to decline to just one tenth by 2050. Land degradation includes forest loss; while globally
this loss has slowed due to reforestation and plantations it has
accelerated in tropical forests that contain some of the highest levels
of biodiversity on Earth. Ongoing degradation has many impacts on
species, the quality of habitats and the functioning of ecosystems.WWF Living Planet Report 2018 page 6

@DevSolar BTW page 90 of the report is a page 48 of the PDF.
– user31389Oct 30 '18 at 16:19

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Might be worth a warning that the link will download a PDF. "How to interpret the Living Planet Index" is on page 47 of the PDF (pg. 91 of the document), and "Where does the Data Come From?" is on page 49 of the PDF (pg. 94), if you'd like to expand your answer.
– 1006aOct 30 '18 at 19:19

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It sounds like the report is weighing declines in small populations the same as declines in large ones. That's rather different than saying "there are 60% fewer vertebrates alive now than in 1970," which is how I would interpret the headline.
– Aleksandr DubinskyOct 30 '18 at 23:27

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Quoting the primary source of a claim to validate a claim is very bad practice (except in the case where headline reports misrepresent the claim). Reiterating the claim is not skeptical analysis of the claim.
– matt_blackOct 31 '18 at 10:15

The index for deep-sea fish populations for the North Atlantic
(Figure 13) is based on 77 populations of 25 species, and indicates a
72 per cent decline over the last 40 years. In the last two decades the
index is more or less stable, but not showing signs of recovery.

(Source: "The Living Planet Index database. WWF and the Zoological Society of
London. Downloaded 3 March 2015. www.livingplanetindex.org")

One of the problems I have with the story is that it doesn't clearly specify exactly what 60% means or answer the question 60% of what?. Hence the need for good analysis to unpick the campaigning hype from the real data.
– matt_blackOct 30 '18 at 12:40

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@matt_black 60% reduction in population = population today is 60% smaller than population at date X in the past. Example: in 1970 there were 1,000 individuals, today there are 400 individuals of the same species, that's a 60% decline. I don't see what's unclear about that claim.
– gerritOct 30 '18 at 13:00

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@gerrit: Are several dozen chickens less outweighed by one bull more? Are five species extinct outweighed by a five-fold multiplication of another? Would a lifeless continent of Africa be outweighed if Australia would just flourish that much more? Are we only talking about specific populations that have been watched non-stop from 1970 till today, or does it suffice to have measured one population of "birds" in 1970 and another in roughly the same place today? ("Population" is a rather specific term for biologists.) The claim is not about one specific number but a general trend.
– DevSolarOct 30 '18 at 13:07

@jpmc26 Check the edit history of the question. I took the liberty of adding them to the answer before the edit ruled them out explicitly. Also, while not part of the claim itself, the significant reduction observed in insect population lends credibility to similar reductions in other groups of animal life.
– DevSolarOct 30 '18 at 15:02

The guardian article seems to more or less include the data that's in the WWF report.

Similar data is reported in the literature. For instance, this article [1] from Science says

Our analyses suggest that biodiversity has continued to decline over the past four decades, with most (8 out of 10) state indicators showing negative trends (Fig. 1 and Table 1). There have been declines in population trends of (i) vertebrates (13) and (ii) habitat specialist birds; (iii) shorebird populations worldwide; extent of (iv) forest (14, 15); (v) mangroves; (vi) seagrass beds; and (vii) the condition of coral reefs. None show significant recent reductions in the rate of decline (Table 1), which is either fluctuating (i), stable (ii), based on too few data to test significance (iii to vi), or stable after a deceleration two decades ago (vii). Two indicators, freshwater quality and trophic integrity in the marine ecosystem, show stable and marginally improving trends, respectively, which are likely explained by geographic biases in data availability for the former and spatial expansion of fisheries for the latter (5). Aggregated trends across state indicators have declined, with no significant recent reduction in rate: The most recent inflection in the index (in 1972) was negative (Fig. 2). Because there were fewer indicators with trend data in the 1970s, we recalculated the index from 1980, which also showed accelerating biodiversity loss: The most recent inflection (2004) was negative. Finally, aggregated species’ extinction risk (i.e., biodiversity loss at the species level) has accelerated: The International Union for Conservation of Nature (IUCN) Red List Index (RLI), measuring rate of change (16, 17), shows negative trends.

The article mentions some specific data in the referenced table. Key points (data for 1970-2010):

If you are concerned about the methods used to generate the data in the WWF report, see [2]. This covers how data is collected, what data is collected, how data is aggregated, limitations in the data set, population aggregates and divisions. The paper also discusses trends and analysis. As an example, you'll find discussion like the following in the paper:

These issues highlight concerns over indicators of biodiversity change that are more wide reaching. The factor that most limits progress in understanding biodiversity change is that many of the existing data relate to species from temperate regions, whereas the majority of biodiversity is found in the tropics.

In summary, it's not clear to me where the 60% number came from in the original question. But the "big picture" being reported is approximately correct, and the numbers are more or less worse than 60% depending on what populations you consider.

This is not the same as losing 60% of animals, because population sizes vary a lot. The Atlantic illustrates the distinction well with this hypothetical example:

imagine you have three populations: 5,000 lions, 500 tigers, and 50 bears. Four decades later, you have just 4,500 lions, 100 tigers, and 5 bears (oh my). Those three populations have declined by 10 percent, 80 percent, and 90 percent respectively—which means an average decline of 60 percent. But the total number of actual animals has gone down from 5,550 to 4,605, which is a decline of just 17 percent.

The report doesn’t make any claims about absolute vertebrate numbers (as far as I’m aware); it instead attempts to produce an indicator of biodiversity.

"...it states that the average decline of populations of vertebrates is 60%...." To be fair, that is even exactly what they state later on in the article "...populations fell by an average of 60%..". It's mostly the first two sentences of the article that seem to be misleading.
– TrilarionNov 1 '18 at 17:02

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@Trilarion: thankfully, everyone reads right to the end of articles, and ignores earlier claims if they’re subtly contradicted by later claims!
– Paul D. WaiteNov 1 '18 at 17:03

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This is much better than 'my' answer, which is at best just a clarification of what is actually being claimed; thanks for this.
– RogerNov 1 '18 at 18:00

@Roger: well naw, I’m just trying to clarify the report’s claim too, or rather correct the misleading reporting of it. Feel free to add stuff from my answer to yours — I think your primary quoting is helpful, but I think the 60%-reduction-in-animals vs. average-60%-reduction-per-population confusion is the lede.
– Paul D. WaiteNov 1 '18 at 20:57